1. Field of the Invention
The present invention relates to an ink-jet recording apparatus of such a type that recording head means receives ink from a closed blader containing a supply of ink and being made of a flexible material such as a plastic sheet, and an ink tank used therein to house such a closed bladder.
2. Related Background Art
Attention is recently drawn to an ink-jet recording apparatus since it is suitable for monochrome as well as color recordings. For color and gradation recordings, a plurality of ink tanks corresponding to the number of ink colors (for example, yellow, magenta, cyan and black) and the same number of recording heads are required. Besides hue, the use of thick and thin inks also requires a plurality of ink tanks. This is true in either of a serial type ink-jet recording apparatus in which scanning is effected by carriage means or a full-line type ink-jet recording system in which scanning is carried out in each transmission of paper.
In such ink-jet recording apparatus, an ink bladder is used which is made of a flexible material such as plastics sheet, metal-plastics sheet lamination or the like. The ink bladder is replenished with new ink while expelling air out of the bladder. A pair of electrodes are disposed at spaced positions, for example, at upper and lower positions within the bladder such that the residual amount of ink within the bladder will be sensed by measuring changes in the electric resistance between the electrodes. In such an ink sensing method, however, some amount of ink frequently remains between the electrodes even after the ink bladder has collapsed due to reduced amount of ink. This does not provide an accurate detection of ink in the bladder.
To overcome such a problem, a hard vessel may be used to contain ink in place of such a flexible ink bladder. The hard vessel must have a vent to prevent the internal pressure of the vessel from being changed. This provides additional problems in that the ink solvent is evaporated through the vent to change the properties of the ink within the bladder such as viscosity and others and in that the ink tends to leak out of the bladder through the vent.
The ink bladder is normally in the form of "a floor cushion" made of a plastics sheet or the like. Such a form of ink bladder is mounted within the ink-jet recording system in a substantially horizontal position. Therefore, the ink bladder is reduced in thickness to decrease the internal pressure of the ink bladder. In such a case, there are still other problems that the size of the ink vessel is undesirably reduced and that the residual amount of ink in the ink bladder is difficult to be sensed. Particularly, a pair of upward and downward electrodes spaced away from each other to sense the residual amount of ink due to changes in the electrical resistance between the electrodes are substantially difficult to be used.
In an ink-jet recording apparatus including a plurality of ink reservoirs, a pair of spaced electrodes is disposed within each of the ink reservoirs to measure the change of electric resistance between the electrodes, depending on the residual amount of ink within that ink reservoir. In such a case, a set of measuring and sensing circuits are required for each of the ink reservoirs. Therefore, the associated detecting means is costly and complex.